1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine.
2. Discussion of the Background
A two-component developing method is well known as a remarkable developing method especially for high-speed image reproduction using powder type toner, such as electrophotography or the like. In recent years, the two-component developing method is used as a major technology in the copying machine, laser printer, etc.
In the two-component developing method, a developer is conveyed to a surface of a non-magnetizable sleeve in which magnets are mounted inside thereof, which is a developer bearing member, and the developer kept in a brush-like state (magnet-brush) contacts or is positioned adjacent to an image bearing member. Developing is implemented with a toner, which is mixed with the developer, which selectively adheres onto a latent image of an image bearing member by an electric field between the image bearing member on which a latent image is formed and the sleeve to which an electric bias is applied.
However, in a field that requires a high quality image, it is considered to be a problem that image noise, such as background fouling and/or deterioration of a sharpness of an image, due to irregular toner adherence onto the latent image, tends to appear. An image having a uniformity of dots that is formed in an interval of dozens of microns is required in order to smoothly reproduce a halftone image, for example, a printer, a digital copying machine, or the like. However, when the dot image is observed under a microscope, a large dispersion of shapes of dots or area of dots is found and toner is irregularly adhered between the dots. If this phenomenon is significant, roughness cannot be disregarded and thereby the image is less uniform.
To solve the above-described problem, a method for applying a developing bias (hereinafter called AC bias) having a vibration-component is proposed in Japanese Laid-Open Patent Publication No. 6-19213/1994, Japanese Laid-Open Patent Publication No. 7-72699/1995, Japanese Laid-Open Patent Publication No. 7-114223/1995, and the like.
However, in the conventional developing method described in each of above Publications, the quality of the image is not always improved but also deteriorates depending on a condition of applying the developing bias. This has been experimentally confirmed by the inventor. It is assumed that the movement of the toner or the carrier is affected by various factors.
Conditions such as a range of diameters of toner particles and diameters of carrier particles and a range of frequencies of the vibration bias are described in the Japanese Laid-Open Patent Publication No. 7-114223/1995, which indicates that a high frequency is preferably not less than 6000 Hz, and that the volume resistivity of the carrier is preferably not greater than 10.sup.10 .OMEGA. cm to maintain a good developing efficiency.
However, in accordance with an additional test carried out by the present inventor, it is confirmed that a phenomenon of blurring of a trailing edge portion of a solid image (hereinafter called trailing edge blurring) tends to occur in the range of the aforementioned frequency. Further, it is confirmed that, when the frequency of the vibration bias is raised up indefinitely, the effect of applying an AC bias is lost. Even though a slight improvement is confirmed by using a low resistivity carrier, a sufficient image quality is not obtained.
Various types of carriers are known, including a carrier composed of a magnetizable particle coated with resin, which is widely used in order to obtain durability (disclosed in, for example, Japanese Laid-Open Patent Publication No. 6-19213/1994). Further, a non-contacting developing method in which a magnetizable resin coated carrier is used and an AC bias (vibration bias) applied is described in Japanese Laid-Open Patent Publication No. 7-72699/1995.
However, in accordance with an additional test performed by the present inventors problems of image quality exist such that the uLiilo-rnity of the dot area is not sufficient and a phenomenon such as the trailing edge blurring is observed. The following phenomenon is surmised to be the reason that causes the problems.
If the carrier has a high electric resistance, for example, a positive-polarity charge (the opposite polarity to that of the toner) remains on a surface of the carrier when the toner has left from the carrier. On the contrary, if the carrier has a low electric resistance, the positive-polarity charge remaining on the surface of the carrier flows away in a relatively short time period. Thus, when the carrier has a high electric resistance, the charge of the opposite polarity to the toner remains on the surface of the carrier when the toner has left from the carrier. Therefore, the charge remaining on the surface of the carrier is surmised to generate a noise that abnormally attracts the toner appropriately moving towards the latent image on a photoconductive element.
There are some methods for making the magnetizable resin-coated carrier to have a low electric resistance. One of the methods that decreases the electric resistance of the carrier uses a thin coat layer and has a problem in regard to prolonging the life of the developer. As another method, a material having a low electric resistance may be used as a coat layer itself. Practically, a carrier composed of a ferrite or a magnetite as a core material, and silicone resin or the like as a coat layer in which carbon or the like is scattered can be used, since such a carrier is inexpensive and stable.
However, when the present inventor carried out developing by using the aforementioned carrier, i.e., a carrier composed of a ferrite or a magnetite core, and silicone resin or the like as a coat layer, in which carbon is scattered applying AC bias, a phenomenon that the toner image has small white flecks (hereinafter called white flecks) where the toner is removed is observed.
After repeated experiments, the inventor found that if peak-to-peak voltage of the AC bias is decreased, the less white flecks occur, and only at the toncr-adhered part of the image (the part where the potential difference from the peak potential of the AC bias is relatively large), and not when the carrier having the layer in which the carbon is not scattered is used, and the like. Consequently, it is surmised that the phenomenon of white flecks is caused by a discharge from the carbon adjacent to the surface of the carrier to the image bearing member.